The exemplary embodiments of this invention relate generally to bio-sensors and, more particularly, to bio-sensors based on field effect transistors.
Sensors based on field effect transistors (FETs) can be used in a variety of different bio-sensing applications to detect various biomolecules. In such sensors, a sensing surface is modified with a functional group that acts as a receptor to bind a target species having a charge. The target species may be any biomolecule such as a protein, virus, drug moeity, or the like. The charge of the bound target species on the sensing surface causes change in the drain current that can be used in a bio-sensing application.
The sensitivity of a FET-based bio-sensor is generally limited due to the probability of an unbound target species attaching to the sensing surface. In the case of typical sensors, both sensing and non-sensing (also known as passive) surfaces are of the same material (i.e. SiO2) and therefore have the same chemistry. Hence, an unbound target species in a solution can bind to both sensing and passive surfaces. The sensitivity depends on the ratio Asense/Apassive where Asense is the sensing surface area and Apassive is the passive surface area. Since the passive surface area is significantly larger than the sensing surface area (e.g., generally on the order of 103 to 104 times as large) and since the surfaces are of the same chemistry, the majority of target species would attach to the passive surface and not to the sensing surface. Based on the construction of bio-sensors of this type, a particular molecule of a target species in dilute concentrations may not be detected due to the low probability of such a molecule binding to the sensing surface, thereby providing inaccurate detection readings. Consequently, having sensing and passive surfaces of disparate surface areas and of the same material limits the sensitivity of a FET-based bio-sensor.